A semiconductor module of this type is known for example from R. Zehringer et al., “Power Semiconductor Materials and Devices”, Materials Research Society Symposium Proceedings, Volume 483, 1998, pages 369-380. This publication describes a semiconductor module with a module housing, a metallic base plate and a plurality of semiconductor elements, in this case IGBT (Insulated Gate Bipolar Transistor) chips and diodes, arranged on said base plate and covered by said module housing. The module housing is generally filled with a silicone gel composition, which serves as an electrical insulating layer and as corrosion protection and also reduces tensile forces acting on connecting wires. The base plate is connected to a water cooling arrangement, to dissipate the heat generated by the semiconductor elements. Arranged on the base plate is a substrate in the form of a metal-coated ceramic board. It has an electrical insulation between the semiconductor elements and the base plate or water cooling arrangement and, moreover, has good thermal conductivity, to dissipate the heat of the semiconductor elements to the base plate. The base plate, ceramic board and semiconductor elements are soldered on one another, the metal layers of the ceramic board permitting the soldered connection.
Good thermal conductivity and poor electrical conductivity can nowadays be combined in materials, so that there is no difficulty in producing insulating elements which are relatively thin but conduct heat well, for example from aluminum nitride (AIN), with a good electrical insulating capacity. For instance, a thickness of 1.5 to 2 mm is theoretically adequate to insulate 20 kV.
Edge effects, caused in particular by edges and corners of the metal layers, adversely affect the dielectric strength of the semiconductor module, however, in particular in the case of high-power semiconductor modules above 1.2 kV. The edges and corners of the metal layers have an inhomogeneous, intensified electric field. This excessive field increase leads to partial discharges and limits the dielectric strength of the entire construction. In this case, the field strength at the edges is at least the square of the voltage, with the result that massively thicker electrical insulation would be necessary to avoid such partial discharges. Air bubbles that may be produced precisely in the edge zones when gel is filled into the module housing are conducive to partial discharges and constitute an additional critical factor with regard to the functionality of the semiconductor module.
There are various approaches to solving this insulation problem. In DE 199 59 248, clearances are formed in field-critical regions and filled with gel, consequently forming an additional interface which prevents the spread of discharges. In EP 1 041 626, the field is reduced in critical regions by three-dimensional rounded portions in the substrate. Both solutions are complex and expensive to produce.